Mechanical stresses are known to play a role in charge carrier mobility which affects Voltage threshold shifts. The effect of mechanical stresses to induce a strain on an FET channel region and thereby influence charge carrier mobility is believed to be due to complex physical processes related to acoustic and optical phonon scattering.
Generally, manufacturing processes are known to introduce strain into the MOSFET device channel region. For example, some strain (stress) is typically introduced into the channel region by formation of an overlying polysilicon gate structure and silicide formation processes. In addition, ion implantation and annealing processes following formation of the gate structure typically introduce additional stresses into the polysilicon gate structure which translate a strain into the underlying channel region altering device performance.
Prior art processes have attempted to introduce offsetting stresses into the channel region by forming stressed dielectric layers over the polysilicon gate structure following a silicide formation process. These approaches have met with limited success, however, since the formation of the stressed dielectric layer of a particular type of stress e.g., tensile or compressive, has a degrading electrical performance effect on a CMOS device formed to operate with an opposite type of majority charge carrier (e.g., NMOS vs. PMOS). For example, as NMOS device performance is improved, PNMOS device performance is degraded.
Other shortcomings in prior art approaches are the adverse affect of the dielectric stressed layers on subsequent gap filling ability of a subsequent inter-layer dielectric (ILD) layer deposition. For example, the thickness of the dielectric stress layer, and therefore the stress altering influence, is limited due to the formation of narrower gaps between devices, a limitation that will increase as device sizes and gap sizes between devices decreases. For example, increasing the dielectric layer stressed layer thickness over one device to increase a stress to improve a charge carrier mobility has the offsetting effect of degrading an opposite polarity FET device performance including drain current (Idsat).
These and other shortcomings demonstrate a need in the semiconductor device integrated circuit manufacturing art for improved strained channel FET devices and methods for forming the same to selectively control an induced stress type and level to improve both NMOS and PMOS device performance and reliability.
It is therefore an object of the present invention to provide improved strained channel FET devices and methods for forming the same to selectively control an induced stress type and level to improve both NMOS and PMOS device performance and reliability, in addition to overcoming other shortcomings in the prior art.